If you come on the mat impaired, well, I'm with George -- you should find the door asap. I don't care why you're impaired or what you're impaired from -- I don't want you hurt nor do I want you doing anything to or with anyone else. If you're not 100% there do everyone a favor and stay home. Enjoy it in private, but I don't want you cranking on my elbow...

Other than that, well, it kinda depends on the answer to the "to what end" question, neh? Few really doubt there can be medical reasons to use marijuana. The problem is that like some drugs it can impair the user. So see paragraph 1 up above...